Multilayered superconducting circuits are fabricated using a variety of fabrication steps including vacuum deposition, photolithography, etching and more. In vacuum deposition, a host material, a portion of which is to be deposited in a thin film on a substantially planar substrate material, is caused to emit or provide a stream or flux of atoms of the host material. (In sputtering, for example, atoms from the surface of the host material are ejected by bombarding the host material with energetic particles). The flux of atoms emitted from the host material and arriving at the substrate arrives with an angular distribution.
In some of the layers of deposited thin film it is desirable to form a pattern by removing the material of the thin film in some regions and leaving the thin film in place in other regions. To accomplish this, a photoresist material is uniformly distributed over the surface of the thin film. A mask containing the pattern to be transferred to the thin film is placed above the thin film. Light is shown through the mask which light dissociates the photoresist wherever it impinges on the photoresist. The photoresist not developed by the light is washed away in a special bath but the unexposed photoresist remains. Then the surface of the thin film is etched and that portion of the thin film not protected by the unexposed photoresist is removed. Following this the unexposed photoresist is removed in a special bath leaving behind the pattern of the mask in the thin film. This process of transferring the pattern to the thin film is called photolithography. The pattern comprises plateaus of material from the top thin film and lower areas of previously deposited thin film layers or the substrate itself which lower areas are exposed by the process of photolithography. The difference between a lower area and a plateau is called a step. Note the process described above uses a commercially available positive photoresist such as the Shipley AZ 1350J. A similar process can be used with a negative photoresist such as the Kodak 809.
A problem arises when it is necessary to deposit new thin film layers over the recently patterned thin film, or more particularly, over the steps. Because the atoms of the host material forming the thin film arrive at the substrate with an angular distribution and because the steps have substantially vertical walls, the plateaus act to shadow the lower areas and prevent the atoms forming the new thin film from growing at the same rate in the lower areas as on the plateaus. As more and more material is deposited over the steps, a crack may develop in the thin film layers over the steps, as the films grow together. This destroys the usefulness of the device.
The present invention overcomes the problem of cracking when covering steps with layers of thin films by providing for the fabrication of patterned planer layers over the substrate, formed in part by anodizing portions of thin metallic films including anodizing all of preselected portions of the metallic film completely through the thickness of the film to the underlying thin film when necessary. This in part eliminates the steps.
In the process of liquid anodization, a metallic member to be anodized is connected to the positive terminal of a power supply and the metallic member is immersed in an electrolytic solution. The negative terminal of the power supply is connected to a cathode which is also immersed in the electrolyte. In another form of anodization, plasma anodization, an oxide film is grown on a metallic member by applying an electric field between the metallic member and the oxygen plasma in which it is immersed. As long as current flows from the cathode through the electrolyte (or oxygen plasma) and then through the metallic member to the positive terminal, oxidation of the metallic member occurs. However, with prior art anodization (both liquid and plasma) it has been difficult to fully anodize all portions of the metallic member which are to be anodized due to nonuniformity in the thickness of the films. Internal metallic regions of the metallic member become electrically isolated due to the formation of surrounding, insulating areas of anodized metallic material. Once these internal regions become electrically isolated from the positive terminal, it is no longer possible to anodize them. Therefore, the article may contain unwanted, unanodized, interior metallic regions, which detract from the usefulness of the anodized metallic member for its intended purpose. Also, the presence of unanodized metallic regions reduces the effectiveness of the anodized material as an electrical insulation medium. The present invention overcomes this problem with an improved method of anodization.
The problem discussed above with respect to anodization is described in Romankiw, U.S. Pat. No. 3,971,710. See column 1, lines 18-44. Romankiw discloses an anodizing process which includes depositing a first conductive layer on a dielectric layer and then depositing a porous metal oxide-forming layer on the first conducting layer. Then the porous metal oxide-forming layer is completely anodized and the first conductive layer is treated to render it non-conductive. However, Romankiw also discloses that when attempting to render the first conducting layer entirely non-conductive after anodizing the porous metal oxide-forming layer so that both the first conductive layer and the porous metal oxide-forming layer are rendered non-conductive through the complete thickness to the dielectric substrate, the electrically conductive layer may still conduct electricity up to a very small degree, "such as when the bulk of the underlayer (electrically conductive layer) is rendered nonconductive with small amounts of untreated metal residue hydroxides or hydrated oxides remaining." See Col. 7, lines 34 through 42. Hence, Romankiw discloses a method of completely anodizing a porous oxide-forming conductive layer using a first conductive layer in contact therewith, but does not disclose how to eliminate the aforementioned prior art problem in anodization when attempting to render both the porous metal oxide-forming metal layer and the first conductive layer completely nonconductive through the complete thickness of both layers to the underlying substrate.